This invention relates to an ionization detector which ionizes a sample gas by radiation from an electrical discharge and which detects the ions in the form of a current. Photoionization detectors can be classified into at least two broad types. In the first type, a sample gas is directly ionized by light radiation. In the second type, known as an electron capture detector, a gas different from the sample gas is ionized by light radiation and an electrophillic sample gas captures electrons from the ionized gas to neutralize it.
The ionization detector is used not only as a detector for the effluent sample gas from a gas chromatograph, but also as an ion source in a mass spectrometer and as a light source in a photoelectron spectrometer.
An ionization detector is composed of a discharge section which emits radiation by an electric discharge, a collector electrode section which collects ionic currents by a sample gas ionized by the radiation, and a container which holds the sections together. In, for example, a prior art apparatus as disclosed in U.S. Pat. No. 3,454,828, a spark discharge is utilized as the source of radiation. More specifically, two pointed electrodes define a gap of between 2 to 3 millimeters, and the spark discharge arises in this gap. The spark discharge is attended with wandering of discharge on the electrode surfaces and fluctuations of discharge power, which are causes for the generation of noise. As the result, the signal-to-noise ratio is lowered and the sensitivity of the ionization detector is degraded. As is well known, the spark discharge conforms with the Paschen's Law. In order to keep the luminous intensity constant and stable, therefore, the gap between the electrodes must be maintained constant. The electrodes, however, are consumed, for example, by sputtering so that the gap between the electrodes changes. The change appears in the form of a change of the luminous intensity in the discharge. That is, the luminous intensity becomes unstable. In consequence, the sensitivity of the ionization detector becomes unstable.